The present invention relates to a method and apparatus for laying a marine pipeline, in particular to a method and apparatus for installing accessories on the end of a pipeline and/or at one or more intermediate locations along the pipeline.
Marine pipelines for conveying gas or crude oil are typically laid from specialised pipelaying vessels or barges, whereby pipelines are deployed and lowered to the seabed in a controlled fashion, the pipelines either being assembled from individual pipe sections on the vessel (referred to as “stovepiping”) or spooled from one or more reels mounted on the vessel (referred to as “reel pipelaying”).
The stovepiping method requires the pipelaying operation to be periodically halted to permit additional pipe sections to be welded to the pipeline and also requires many experienced welders and additional welding equipment to be carried on the vessel, working in difficult conditions. Therefore stovepiping is a relatively slow and difficult process.
By contrast, reel pipelaying, wherein the pipeline can be prefabricated onshore and spooled onto a large reel, reduces the labour requirements onboard the vessel and considerably speeds up the pipelaying operation. However, a requirement of the reel pipelaying method is to provide means for straightening and guiding the pipeline as it leaves the reel, typically by means of series of rollers or tracks that impart sufficient reverse bending force to remove residual curvature from the pipeline and guides the pipeline into the water at an angle optimised to reduce bending stresses.
For both stovepiping and reel pipelaying, there are two main methods of guiding the pipeline from the vessel, namely S-lay and J-lay.
According to the S-lay technique, the pipeline is passed over guide means as it leaves the vessel so that it is made to follow an S-shaped path with a very large radius of curvature so as to prevent any plastic deformation. The pipe leaves the vessel at a very oblique angle, with the aid of an inclined arcuate ramp known as a stinger. This technique is suited only to shallow and moderate laying depths because otherwise the weight of the pipeline already laid would run the risk of bending and plastically deforming the pipe at the point where it leaves the stinger. In order to lay pipeline at greater depth it is necessary to increase the pipeline water entry angle.
According to the J-lay technique, the pipeline is guided down a vertical or steeply inclined ramp on the laying vessel so that the pipeline leaves the vessel substantially vertically, whereby the pipeline can immersed to a great depth, still without plastic deformation. Thus the J-lay technique is suited to laying pipeline in deep water.
WO 93/06401, WO 93/06402 and WO 95/22484 disclose known pipelaying vessels provided with a pipe take-off assembly mounted adjacent to the stern of the vessel. The take-off assembly includes a pipe take-off or guide ramp which contains straightening and tensioning devices as well as additional pipe clamping means. The take-off ramp is rotatably journalled to permit adjustment of the inclination of the ramp to permit variation of the pipeline water entry angle from about 20° to about 90° for laying pipeline from depths as shallow as 60 metres to much greater depths. The upper part of this range, from about 60° to about 90°, can be used for deep-water laying in 1,000 metres and greater depths.
Thus the vessels can be used to lay pipeline from a reel onto the seabed at a wide range of depths.
During a pipelaying operation it is necessary to attach accessories to the ends of the pipeline and/or at one or more intermediate locations along the pipeline. These accessories are usually quite large and heavy and are commonly designed to be welded to an end of the pipeline, requiring interruption of the laying of the pipeline. Examples of such accessories are initiation fittings, pipeline end terminations (PLETs), pipe valves and tee assemblies.
A problem with such accessories, particularly with the reel pipelaying method, is that they will not readily pass over the take off ramp and thus cutting of the pipeline and installation of the accessory must normally be done downstream of the take off ramp, requiring clamping of the end of the pipeline and halting of the pipelaying operation (and halting of movement of the pipelaying vessel) while the accessory is installed. Various prior art solutions are known to mount an accessory at the end of a pipeline or at an intermediate location in or on a pipeline, such as those shown in WO03/067019, WO 2006/089786 and WO 2006/054891, although each have disadvantages, in particular requiring a considerable interruption to the pipelaying operation while the pipeline is cut and the accessory is moved into position and connected to the end of the pipeline.
For example in WO 03/067019 a system for introducing an inline accessory into a pipeline is described. The pipelaying system comprises a pipeline spool, a tensioner and a clamp, usually referred to as “hang-off” clamp. The hang-off clamp is adapted to hold the end of the pipeline and support the weight of the launched pipeline while the accessory is moved into position and connected to the pipeline.
The tensioner is mounted on a take off or launch ramp, having variable inclination. The hang-off clamp is capable of translating horizontally, while the launched pipeline is suspended therefrom, between a pipeline feeding position beneath the launch ramp and an accessory connection position spaced to one side of the feeding position. Means are provided for lowering the pipeline and accessory connected thereto past the clamp.
The need to laterally translate the hang-off clamp increases the risk of dropping the end of the pipeline, requires complex actuating mechanisms that must operate in adverse conditions while supporting the weight of the pipeline and thus exposed to heavy loads and causes substantial delay in the pipelaying operation when an accessory needs to be connected to the pipeline.